This project is concerned with the neurophysiology of cellular learning. Previosly, we have demonstrated classical conditioning of the pupillary dilation reflex in the muscle-relaxed, unanesthetized cat, using acoustic and electrocutaneous stimulation as the conditioned and unconditioned stimuli, respectively. Additionally, we have reported enhanced cellular discharge to the conditioned stimulus (cellular learning) in the auditory system under conditions which rule out possible changes in effective stimulus intensity and non-specific changes in neural excitability. Furthermore, we found that cells in the magnocellular division of the medial geniculate nucleus exhibit learning while those in the adjacent ventral division of this same thalamic auditory nucleus do not. The magnocellular neurons develop extremely rapid conditioned responses; moreover, a pronounced onset discharge to the initial presentation of the conditioned stimulus is predictive of rapid and relatively persistent cellular learning. As such a predictor is highly unusual, we will extend these studies to the various subdivisions of the auditory cortex to test its generality. Also, the magnocellular medial geniculate will be examined to determine whether or not it is a site of active (local) learning. Finally, we will study the physiological properties of the magnocellular and ventral divisions of the medial geniculate in the absence of classical conditioning training to determine the variety of circumstances under which physiological plasticity is expressed.